A variety of diverse pumping applications use a submersible pump to pump fluid from a container or geological formation. Examples include pumps for well water and sump pumps as well as pumps used in various commercial applications, such as those used in hydrocarbon or oil wells. These pumps may include a manual or automatic switch or sensor to activate the pump to remove fluid from a man-made or naturally formed container, depression, tank, well, formation, etc. Automatic operation of one or more pumps may be based on the level of fluid to control the level of fluid within a desired range. For example, a float switch may be used to automatically activate a pump when fluid level rises above a designated high level, and deactivate the pump when fluid level is below a designated low level. Other sensors may be used to control pump operation based on pressure, temperature, acoustic or optical depth detection, etc. depending on the particular application.
Many submersible pumps are powered by a closely coupled motor that powers the pump and may rely on the fluid to cool the motor during operation. As such, the motor can be damaged if the pump is operated without sufficient fluid for cooling, sometimes referred to as operating dry. In addition, low fluid levels may adversely impact pumping performance if air is entrained within the fluid (cavitation) and could also damage pump components. Submersible pumps are often difficult to service and/or replace as they are often deployed in relatively inaccessible locations. In addition, inoperative pumps may result in costly delays in commercial applications, or related damage if the fluid floods surrounding areas of the container/well. As such, it is desirable to have a reliable device to detect the fluid level, which may then be used to control pump/motor operation.
Submersible pumps and related equipment used in oil wells and similar applications may have relatively unique requirements associated with the significant depths of the wells, the characteristics of the fluid (such as viscosity, contaminants or sediment, volatility, combustibility, etc.), and the potential cost associated with repairing or replacing an inoperative pump/motor. As described above, the fluid level in a container or well may be important for at least two reasons. First, many submersible pumps provide cooling by forcing the fluid being pumped around the motor windings to cool the motor. Insufficient fluid flow or level may cause the motor temperature to rise resulting in a thermal overload condition and possibly permanent damage of the motor or pump components. For oil well applications, the fluid is generally a mixture of hydrocarbons including oil, in addition to water and sediment. Because oil is lighter or less dense than water, it is generally desirable to lower the fluid level in an oil well to extract as much oil as possible. The higher the fluid level in an oil well, the more water that will be present in the cut of crude oil.
There are a number of commercially available optical, electromagnetic, and acoustic or echo devices that are currently being used to measure fluid level in a well. However, these devices are essentially stand-alone devices that are not integrated into the pump/motor control system and require manual measurements. This takes time out of a production facility operator's schedule and reduces productivity of the well, with associated opportunity cost to the well owner. In addition, the desired fluid level for a particular well may change over time as the source is depleted. Once the desired fluid level for a well is determined, periodic checks are performed to maintain the productivity of the well.
Existing strategies for maintaining a desired fluid level within the well include selecting the pump capacity based on the observed output of the well using periodic manual measurements, or using a variable speed drive (VSD). A VSD changes motor speed and consequently the flow from the pump. A VSD allows a well operator to adjust the motor speed and pump flow as the well productivity changes over time, which may be due to source depletion or the geologic zone closing or becoming clogged over time, for example. Many attempts to regulate a VSD to control fluid level in oil well applications have failed due to various application-specific factors. One approach having limited success uses a pressure transducer at the pump to measure fluid pressure at the inlet, with higher pressure corresponding to more fluid above the pump. However, pressure often changes from zone to zone and at different depths, which requires an operator with sufficient experience to adjust the VSD accordingly. Currently available pressure transducers also have limited life, likely due to the harsh operating environment, and often fail long before a submersible pump motor such that an alternative method (usually manual) must be used to control the fluid level for the remaining life of the submersible pump/motor.